The present invention relates, in general to pneumatic control or regulating valves, and, in particular, to a new and useful valve arrangement having a servomotor, a position controller with a rotatable lever and a valve rod engaged with the servomotor and lever.
German Patent No. 31 16 640 discloses pneumatic control valves comprising a pneumatic servomotor or motor operator, a position controller with pneumatic output, a valve rod between the servomotor and a valve cone or face of the valve which is arranged in a valve housing, an articulated device between a rotatable lever of the position controller and the valve rod, which lever senses the lift position of the valve cone, and a yoke, which securely connects the servomotor with the valve housing. The position controller ensures that a given assignment or allocation for the valve cone position, acting as a controlled variable for an actuating signal is used as a command variable. The position controller compares the actuating signal coming from a pneumatic or electropneumatic regulating or control device with the lift of the valve cone and, when the assignment of the actuating signal to the valve cone lift is faulty, changes the pressure in the servomotor, which preferably operates pneumatically, until the valve cone is in the given relation to the actuating signal.
In order to meet this object, the position of the valve rod relative to the valve housing must be represented or mapped, and a distinction must be made between two groups of position controllers.
In the first group, which generally works according to force comparison, the end of a usually helical spring, or an equivalent structural component part, is securely connected to the valve rod, while the other end is in a working connection with a comparison diaphragm, upon which the air pressure representing the actuating signal also acts. When there is an imbalance between the forces acting upon the comparison diaphragm, an actuator and an amplifier are controlled in such a way that the pressure on the pneumatic servomotor assumes exactly the value with which the desired motor position is constrained, regardless of external, error inducing forces on the valve cone, etc., e.g. friction or flow forces. See German patent application No. P 36 37 068.1. In the following this type of arrangement is referred to as a position controller with translational input.
In addition, position controllers are known which comprise a rotatable lever which is entrained by the valve rod. The angle of the lever relative to an off or neutral position, is a representation of the valve lift. See German patent No. 31 16 640. This group of position controllers operates frequently, but not always, with path comparison. They will be designated in the following as position controllers with rotational inputs.
Positional controllers with rotational inputs are found substantially more often than those with translational inputs, particularly because they can be more easily adapted to the geometric properties of the various pneumatic control valves.
Both groups of position controllers, particularly when used in heavy-duty operation in chemical plants, have the disadvantage that the parts located outside of the position controller housing, namely levers with valve rod articulation or representation springs, can be damaged easily. Such damage comes about because these parts are often exposed to intensive corrosion or because, when the servo-devices are relacquered, these parts are lacquered along with them, so that the position controller is maladjusted. Finally, there is the risk of accident by contact with a person, particularly if the on-site conditions are cramped.
In connection with the position controllers with translational input, mentioned above, it has already been suggested to accommodate representation springs in a closed housing which is generally ventilated.